As cellular communications become more widely used, the number of individual users and calls multiplies. Increase in cellular communications utilization magnifies the opportunity for interference between the different users on the cellular system. Such interference is inevitable because of the large number of users and the finite number of cellular communications cells and CDMA code channels which are available.
This invention applies to Code Division Multiple Access (CDMA) system, where the users would be separated from one another, either using different codes and/or different time delays of the same code, while utilizing the same frequency band. Because of this use of the same frequency band, there is a potential, as the system becomes loaded with a number users, of heavy traffic interference between one user and another limiting the capacity of the system. The comparable system for analog would have users separated on different frequencies with reuse of the same frequencies provided for with a guard distance or guard zone (reuse distance) between points in which the same frequencies are used again. There are certain problems that are inherent to CDMA networks including interference from one cell to another, since typically every cell reuses the same frequency. The forward link at any particular mobile's location may receive interference from a number of cells. Some of those would be desired cells that the mobile would be in handoff with. Others would be cells that the mobile could not be in handoff with, but that would interfere with the signal that the mobile was receiving. An analogous problem happens on the reverse link, where a cell site would receive signals from a number of mobiles, the desired mobiles that are within the coverage area of that sector, as well as mobiles that are being served by other cells, that interference would limit the capacity of a given sector.
To reduce the interference problems caused by other users in the omni cell 360.degree. configuration, cells have been broken down into 120.degree. sectors such that each channel available at the cell only communicates in an area of 120.degree. radial coverage about the cell. An advantage, in addition to the reduction of interference realized by the sector system, is that such a cell achieves extended range as compared to an omni cell 360.degree. system simply due to the ability to focus a greater signal gain on the antennas. Individual cells may then cover a larger area, and communications signals may be stronger within the cell.
It shall be appreciated that loading of sectors is often cyclic or dynamic in nature rather than constant. For example, during certain times of day, such as business commuting times, a particular sector, such as a sector encompassing an urban highway, may service more users than during other times of day. Therefore, during particular times a particular sector or sectors may require increased capacity in order to service all users whereas at other times the cell's capacity might be better utilized when spread more homogeneously throughout the cell's coverage area.
It would, therefore, be advantageous to make more efficient use of cellular capacity by being able to make sectors dynamically shapable in order to provide increased capacity to a particular area within the cell's radiation pattern by making more channels potentially available to that particular area. Ideally, the shapable sectors will be composed of narrow beams so as to provide a convenient means by which sectors may be sized radially about the cell. Systems implementing such narrow beams are described in U.S. Pat. No. 5,563,610, entitled "NARROW BEAM ANTENNA SYSTEM WITH ANGULAR DIVERSITY," incorporated herein by reference, and the associated above-referenced co-pending and commonly assigned continuation-in-part U.S. patent application entitled "APPARATUS, SYSTEMS AND METHODS FOR MULTIPLE ANTENNA TRANSMISSION IN WIRELESS COMMUNICATIONS SYSTEMS." Management of such a system, including concurrent beam and channel management within a neighborhood of cells, is disclosed in the above referenced co-pending and commonly assigned U.S. patent application entitled "METHOD AND APPARATUS FOR IMPROVED CONTROL OVER CELLULAR SYSTEMS."
Another problem in the art is that in a cellular system, communications are typically mobile, often in vehicles travelling at considerable speed. Such mobile communication devices tend to travel through the various sectors and/or cells of a cellular system, thereby continuously effecting signal quality as fringe or shadow areas are entered and exited. These effects of signal quality are not limited to the mobile communication device itself, but also effect other communication devices operating in the area. For example, a communication device operating in one cell, although experiencing acceptable signal quality itself, may in fact be causing interference for another communication device. Such interference may be in the form of frequency reuse interference, near/far problems, increased energy density and the like. Therefore, it is desirable to provide a means by which such a communication device may be handed off to another sector or cell, although its communication parameters do not necessitate the handoff, in order to better serve another communication device. Likewise, such a communication device may be experiencing communication of a quality so as to be within acceptable parameters although communication of a better quality may be had through an adjacent sector or cell.
One benefit or use of this changing of the sector size would be to load balance the traffic among the sectors of a cell or sectors of adjacent cells. A particular example would be a case where one sector was at a capacity limit, such as either running out of transmit power or being unable able to support any additional users. Other sectors on that cell may have additional capacity to spare, by mapping sectors to beams in a more optimum way, could equalize the load across the sectors and alleviate the overload condition on the sector that previously had reached a capacity limit.
Recognizing the mobility of communications and the attendant communication quality issues, therefore, it would also be advantageous to be able to dynamically shape sectors in their longitudinal, or outboard, reach from a cell site. Preferably, as it is determined that a communication device is causing interference for another communication device or as it is determined that this communication device may itself be better served by another sector or cell, the shape of the sector currently serving the communication device may be adjusted to force a handoff of the communication device to another sector or cell. Likewise, where capacity remains in a sector of any adjacent cell, the sector at capacity could reduce its area of influence simultaneously with the adjacent sector increasing its area of influence, in order to provide additional capacity within the area originally serviced by the sector that previously had reached a capacity limit. Ideally, the longitudinal shape of sectors will be accomplished through the use of attenuators in the receive signal path and equivalent gain adjustments of transmit power in the transmit path.
A need therefore exists in the art for a system and method for dynamically adjusting the shape of cell sectors to provide for greater trunking efficiency and the ability to serve more users. Moreover, a need in the art exists for such a system to provide azimuthal as well as longitudinal shaping of the sectors.